The present invention relates to a continuous process for producing ethylene terephthalate polyesters.
By the term "ethylene terephthalate polyesters" referred to herein we mean those polyesters predominantly comprised of combined ethylene terephthalate units. The term "a polyester-forming fusion melt predominantly comprised of bis-2-hydroxyethyl terephthalate or its low polymers" used herein means a fusion melt suitable for use in the production of an ethylene terephthalate polyester. Other units or components which may be present in the fusion melt to produce copolyesters are well known in the art, and include, for example, bis-2-hydroxyethyl isophthalate, 2-hydroxylethyl p-2-hydroxyethoxybenzoate, and the like. While the description given herein is mainly directed to the production of poly(ethylene terephthalate) it will be appreciated that the invention also contemplates the procuction of other polyesters predominantly comprised of combined ethylene terephthalate units.
There are two routes to poly(ethylene terephthalate); one route comprising the transesterification of dimethyl terephthalate with ethylene glycol (hereinafter referred to as EG), followed by the polymerization of the resultant bis-2-hydroxylethyl terephthalate or its low polymers (hereinafter referred to as BHET), while another route comprising the direct esterification of terephthalic acid (hereinafter referred to as TPA), followed by the polymerization of the resultant BHET. The polymerization may be carried out either batchwise by heating the BHET in one single reaction vessel under an increasing degree of vacuum (i.e. decreasing residual pressure) until the desired degree of polymerization is achieved, or continuously by-passing the BHET in sequence through a series of reduced pressure polymerization vessels maintained under steppedly decreasing residual pressures to obtain a product having the desired degree of polymerization from the last vessel. Early commercial production of ethylene terephthalate polyesters was carried out by a combination of transesterification and batchwise polymerization. Recently a technology of direct esterification of TPA has been advanced and as a result, ethylene terephthalate polyesters have begun to be commercially produced by processes involving direct esterification of TPA, followed by batchwise or even continuous polymerization.
One of the advantages of the continuous polymerization resides in the fact that it enables the continuous production of polyesters in melt form, which may directly be spun into filaments without the additional steps of being extruded into strands, chilled with water, cut into pellets, dried and then melted again, as required in the batchwise polymerization. Furthermore, an additional residence time is generally required in the batchwise polymerization, during which time the product must be held in melt form before being extruded into strands and during which any undesirable reaction under heat can degrade the quality of the product. With the continuous polymerization such an additional residence time can be eliminated or at least substantially reduced. Accordingly, the continuous polymerization is preferred in this regard, even when it is not directly coupled with the spinning process.
In the batchwise polymerization wherein BHET is heated in one single vessel under an increasing degree of vacuum, the desired degree of polymerization of the product may readily be controlled simply by detecting a melt viscosity of the material which is proportional to the degree of polymerization, by e.g., observing a reverse force exerted on a stirrer, and breaking the vacuum when the desired melt viscosity is reached. Whereas, in the continuous polymerization wherein the degree of polymerization of the product is gradually increased during the passage of the polymerizing material through a series of polymerization vessels and it takes a certain period of time for the material to pass through each vessel, the degree of polymerization can be kept at the desired level only after due consideration of these factors, and cannot be as easily controlled as in the batchwise polymerization. With feed-back cascade control wherein only a melt viscosity of the material leaving each vessel is detected and a set point or command value of a manipulating factor or pressure control thereof, for example, a degree of vacuum in said vessel is reset by a controlling system which operates to control the degree of polymerization in cascade, the degree of polymerization in the continuous polymerization process cannot be controlled precisely if there is any variance of the melt viscosity of the material leaving a vessel just upstream of said vessel. This is because the polymerizing material takes a certain period of time to pass through said vessel and, therefore, there is a corresponding time lag until said variance manifests itself as one of the factors contributing to a variance of the melt viscosity of the material leaving said vessel.